Substitutional doping effectively modulates carrier polarity of semiconducting two-dimensional (2D) transition metal dichalcogenides (TMDs) like MoS2. Although Fe doping typically induces n-type conductivity in monolayer MoS2, anomalous p-type behavior has also been experimentally reported, the origin of which remains unresolved. Here, we prove that this anomalous p-type conductivity originates from defect associates formed through interactions between Fe dopants and S atoms, which consists of three Fe substituting Mo (FeMo) point defects arranged into an equilateral triangle with a central S atom, denoted as 3FeMo-S associate. Its p-type effect is directly verified through scanning tunneling microscopy/scanning tunneling spectroscopy (STM/STS) measurement, in sharp contrast to the n-type behavior induced by isolated FeMo point defects, and the conclusion is further supported by electrical transport measurements and first-principles calculations. Similar 3FeW-S associates and their p-type doping effect are also identified in monolayer Fe-doped WS2. This work resolves a longstanding controversy and highlights the critical role of defect associates in modulating properties of 2D TMDs.

Field-tunable reconstruction of crystalline electric field (CEF) doublets offers a promising avenue for inducing multipolar character, while its observation in real materials has been little explored so far. Here we establish the honeycomb rare-earth chalcohalide NdOF as such a platform. Raman spectroscopy identifies four CEF excitations at 1.7, 15.6, 19.2, and 80.9 meV, and a Zeeman–CEF analysis reproduces their nonlinear field splitting into seven branches. Magnetization and susceptibility over 0.1–9 T are well described by a CEF model for the total angular momentum J = 9/2 manifold, confirming the robustness of the extracted CEF scheme. These results demonstrate a field-driven continuous evolution of the ground-state doublet from dipolar to dipolar-multipolar character, with pressure providing a complementary tuning knob, establishing NdOF as a model system for exploring the controlled induction of multipolar components in rare-earth magnets.

The Fe3GaTe2 compound was investigated using dynamic magnetic measurements, microscopic 57Fe M & ouml;ssbauer spectroscopy, complemented by density functional theory (DFT) and Monte Carlo simulations. A reliable fitting scheme was developed for its M & ouml;ssbauer spectra, yielding a complete set of hyperfine parameters from 5 K to above the Curie temperature. The opposite signs of the quadrupole splitting for the FeI and FeII sites indicate that the interactions at the FeI site are predominantly out-of-plane, in contrast to the primarily in-plane interactions at the FeII site. Furthermore, the temperature dependence of the hyperfine field shows that the magnetism of Fe3GaTe2 aligns more closely with the three-dimensional Heisenberg model. The M & ouml;ssbauer spectra also reveal that intrinsic defects enhance spin fluctuations, as evidenced by the presence of a persistent paramagnetic component down to 5 K and significantly broadened linewidths. To understand the origin of these defects, we performed systematic DFT and Monte Carlo simulations. Our theoretical results demonstrate that such defects suppress the ferromagnetic transition temperature and suggest that the observed spin fluctuations are likely linked to FeII vacancies and/or FeII-Ga antisite defects.

As a prototypical transition metal dichalcogenide (TMD) semiconductor, MoS2 exhibits diverse tunable electronic properties in low-dimensional systems, such as Ising superconductivity and charge density waves (CDWs). However, the intrinsic superconductivity of bulk MoS2 remains underexplored. Here, we demonstrate carrier doping in bulk 2H-MoS2 using an ionic-liquid-gating method, resulting in anisotropic bulk superconductivity with a critical temperature T-c of 3.2 K. Notably, the superconducting transition in the bulk requires the lowest critical carrier density (similar to 10(13) cm(-2)) among all the reported superconducting MoS2 systems, while exhibiting a higher T-c than typically observed in monolayers. The electron-phonon coupling (EPC) constant extracted from Raman spectroscopy yields a calculated T-c consistent with the experimental observations, in agreement with the mechanism established in monolayer MoS2. We attribute these observations to the Fermi level preferentially crossing the lower-energy Q point in the bulk, which facilitates superconducting pairing. The results provide deeper insights into the superconducting mechanism in bulk 2H-MoS2.

As the structurally simplest iron-based superconductor, FeSe is a key material for understanding the relationship between electronic nematic order and superconducting order. However, under different experimental tuning conditions, these two ordered states exhibit complex and contradictory interactions described as competition, cooperation, or even decoupling. This review systematically consolidates experimental evidence related to these controversies and examines the potential spin or orbital origins of the nematic order. In conclusion, we present a new physical picture in which the superconducting order and the nematic order may be independently dominated by distinct electronic structures, namely the electron pockets and hole pockets. This provides a new perspective for achieving a unified understanding of the complex phase diagram of FeSe.

The complex interplay between superconductivity, nematicity, and magnetism in iron-based superconductors remains a significant challenge in understanding its high-temperature superconductivity. Despite that numerous experiments aim at revealing the underlying mechanisms for superconductivity and nematicity by varying multiple tuning parameters, the inherent entanglement of these parameters complicates the isolation of the fundamental factors that drive the transitions. Here, by introducing a novel hydrothermal treatment to FeSe, we are able to effectively reduce interstitial Fe without altering the crystal structure and magnetic properties. This treatment results in a notable increase in carrier density and mobility, simultaneously enhancing both superconducting and nematic transition temperatures. Combining our experimental results with previous investigations, we reveal distinct pocket-influenced mechanisms: superconducting order is primarily influenced by electron pockets, while nematic order is driven by hole pockets. The results demonstrate the independent mechanisms of superconductivity and nematicity in FeSe, offering new perspectives on high-temperature superconductivity.

The breathing Kagome material Nb3Cl8 has recently been identified as a single-band Mott insulator and theoretically predicted to host coexisting ferroelectricity, magnetism, and ferrovalley-yet experimental verification of these predictions remains lacking. Here, through polarization-and temperature-dependent Raman and second harmonic generation measurements, we reveal that the low-temperature (LT) phase of this material retains C3 rotational symmetry while breaking inversion symmetry. We propose that the structural phase transition at Ts corresponds to a stacking-order transition: from centrosymmetric AB stacking to noncentrosymmetric AA/BB stacking, driven by enhanced interlayer electric dipole-dipole interactions. With an AA/BB stacking sequence, the LT phase exhibits net electric dipole moments oriented perpendicular to the kagome plane. Our findings provide directly experimental evidence for the existence of a noncentrosymmetric Mott ground state, thereby establishing a unique platform for investigating the interplay between inversion symmetry breaking and strong electron correlation.

Tuning magnetic anisotropy through chemical doping is a powerful strategy for designing functional ma terials with enhanced magnetic properties. Here, we report an enhanced Er3+ magnetic moment resulting from nonmagnetic Lu3+ substitution in the honeycomb-lattice antiferromagnet ErOCl. Unlike the Curie-Weiss type divergence typically observed in diluted magnetic systems, our findings reveal a distinct enhancement of magnetization per Er3+ ion under high magnetic fields, suggesting an unconventional mechanism. Structural analysis reveals that Lu3+ doping leads to a pronounced contraction of the c axis, which is attributed to chemical pressure effects, while preserving the layered SmSI-type crystal structure with space group R3m. High-resolution Raman spectroscopy reveals a systematic blueshift of the first and seventh crystalline electric field (CEF) excitations, indicating an increase in the axial CEF parameter B02. This modification enhances the magnetic anisotropy along the c axis, leading to a significant increase in magnetization at low temperatures and under high magnetic fields, contrary to conventional expectations for magnetic dilution. Our work not only clarifies the intimate connection between magnetism and CEF in rare-earth compounds, but more importantly, it reveals a physical pathway to effectively tune magnetic anisotropy via anisotropic lattice distortion induced by chemical pressure.

Spin-phonon coupling is important in chromate spinel oxides ACr2O4, but its role in LiFeCr4O8 is not well understood. In this paper, we employ Raman scattering and first-principles phonon calculations to study this material. Ten out of 13 Raman-active modes are well assigned. Notably, no phonon splitting is observed across the structural phase transition due to the remarkably small Gr & uuml;neisen constants. This observation, in conjunction with the structural data, provides compelling evidence that the structural phase transition in LiFeCr4O8 is primarily driven by the spin-driven Jahn-Teller effect. Interestingly, some Raman modes (at 207 cm-1, 306 cm-1 and 462 cm-1) exhibit unusual linewidth behavior across the temperature range investigated. Furthermore, the Raman spectra in different phases show no magnetic field dependence. These results suggest that phonons couple with short-range spin correlations, offering insights into how spin and lattice degrees of freedom interact in frustrated systems.

HfRuP and ZrRuAs have garnered significant interest in recent years as potential topological superconductors. However, the mechanism underlying their superconducting pairing, particularly whether it is mediated by electron-phonon coupling (EPC), remains unresolved. Here, we conducted polarized and temperature-dependent Raman scattering experiments to comparatively investigate their EPC. Six Raman-active phonon modes were assigned for each compound based on polarized Raman spectra and first-principles calculations. Temperature-dependent Raman scattering measurements revealed a large and continuous softening of some low-energy phonon modes for both crystals in the measurement temperature range from 1.7 to 300 K. In HfRuP, the E'' (91.5 cm-1) and E' (102.9 cm-1) modes exhibit phonon softening and linewidth broadening upon cooling. In ZrRuAs, the E'' (88.8 cm-1) mode shows similar behavior. Furthermore, the E' mode in HfRuP (357.2 cm-1) and the A'1 mode in ZrRuAs (171.1 cm-1) exhibit conventional frequency hardening but anomalous linewidth broadening upon cooling. Our first-principles calculations attribute these anomalies to a resonancelike enhancement of the EPC between the Fermi-surface electrons and the Ru-dominated E'' phonon mode. The electronic band structures also demonstrate that the enhancement stems from the exact matching in energy scale between the Ru-dominated low-frequency phonon modes and the distribution of their electronic densities of states around EF. These results provide direct evidence that EPC drives the superconducting pairing in both compounds, highlighting the pivotal role of Ru atoms in these ternary transition metal pnictide superconductors.

Realizing Kitaev interactions on triangular lattices offers a compelling platform for exploring quantum-spin-liquid physics beyond the conventional honeycomb lattice framework. Here, we investigate the triangular-lattice antiferromagnet KCeSe2, where multiple probes reveal strong magnetic anisotropy suggesting significant Kitaev physics. Through detailed and combined analysis of magnetization, neutron scattering, and thermodynamic experiments, we identify dominant ferromagnetic Kitaev (K = -1.82 K) and antiferromagnetic Heisenberg (J = 1.34 K) interactions that stabilize a stripe-yz ordered ground state via an order-by-disorder mechanism. Magnetic fields applied along the Kitaev bond direction induce two phase transitions at 1.67 T and 3.8 T, consistent with density matrix renormalization group (DMRG) calculations predictions of a progression from stripe-yz to stripe-canted and spin-polarized phases. Near the 1.67 T quantum critical point, enhanced quantum fluctuations suggest conditions favorable for exotic excitations. These results establish KCeSe2 as a platform for exploring Kitaev physics on triangular lattices.

A substantial energy gap is essential for quantum spin Hall (QSH) insulators in devices and fundamental research. Monolayer 1T'-MoTe2 is recognized as a promising candidate, featuring a potentially large energy gap. However, despite extensive efforts including molecular beam epitaxy growth, mechanical exfoliation, and chemical vapor deposition techniques, achieving stable 1T'-MoTe2 with a well-defined energy gap has proven elusive. Here, we successfully open a band gap of approximately 70 meV in bulk 1T'-MoTe2 by reducing the interlayer interaction through the intercalation of organic cations HMIM+. Both resistivity measurements and infrared spectroscopy confirm distinct semiconducting behaviors. Band structure calculations show that the gap emerges from the absence of interlayer coupling, and that the intercalated 1T'-MoTe2 is topologically nontrivial with Z(2) = 1. This work not only demonstrates a bulk 1T'-MoTe2 possible QSH insulator with a sizable gap, but also provides a different approach to realizing clean, stable platforms for exploring the QSH effect and designing low-power quantum electronic and spintronic devices.

The triangular-lattice quantum Ising antiferromagnet is a promising platform for realizing Anderson's quantum spin liquid, though finding suitable materials to realize it remains a challenge. Here, we present a comprehensive study of NaTmSe2 using magnetization, specific heat, neutron scattering, and muon spin relaxation, combined with theoretical calculations. We demonstrate that NaTmSe2 realizes the transverse field Ising model and quantitatively determine its exchange parameters. Our results reveal a multipolar spin-polarized state coexisting with a dipolar spin-disordered state. These states feature gapless spinon excitations mediated by the multipolar moments. The study shows how multiple types of magnetism can emerge in distinct magnetic channels (dipolar and multipolar) within a single magnet, advancing our understanding of spin-frustrated Ising physics and opening pathways for different quantum computing applications.

Rare-earth chalcogenide compounds ARECh2 (A = alkali or monovalent metal, RE = rare earth, Ch = O, S, Se, Te) are a large family of quantum spin liquid (QSL) candidate materials. NaYbS2 is a representative member of the family. Several key issues on NaYbS2, particularly how to determine the highly anisotropic spin Hamiltonian and describe the magnetism at finite temperatures and the ground state, remain to be addressed. In this paper, we conducted an in-depth and comprehensive study on the magnetism of NaYbS2 from finite temperatures to the ground state. Firstly, we successfully detected three crystalline electric field (CEF) excitation energy levels using low-temperature Raman scattering technique. Combining them with the CEF theory and magnetization data, we worked out the CEF parameters, CEF energy levels, and CEF wavefunctions. We further determined a characteristic temperature of ∼40 K, above which the magnetism is dominated by CEF excitations while below which the spin-exchange interactions play a main role. The characteristic temperature has been confirmed by the temperature-dependent electron spin resonance (ESR) linewidth. Low-temperature ESR experiments on the dilute magnetic doped crystal of NaYb0.1Lu0.9S2 further helped us to determine the accurate g-factor. Next, we quantitatively obtained the spin-exchange interactions in the spin Hamiltonian by consistently simulating the magnetization and specific heat data. Finally, the above studies allow us to explore the ground state magnetism of NaYbS2 by using the density matrix renormalization group. We combined numerical calculations and experimental results to demonstrate that the ground state of NaYbS2 is a Dirac-like QSL.

Recently reported van der Waals layered honeycomb rare-earth chalcohalides REChX (RE = rare earth, Ch = chalcogen, and X = halogen) are considered to be promising Kitaev spin liquid (KSL) candidates. The high-quality single crystals of YbOCl, a representative member of the family with an effective spin of 1/2, are available now. The crystalline electric field (CEF) excitations in a rare-earth spin system are fundamentally important for understanding both finite-temperature and ground-state magnetism but remain unexplored in YbOCl so far. In this paper, we conduct a comprehensive Raman scattering study to unambiguously identify the CEF excitations in YbOCl and determine the CEF parameters and wave functions. Our Raman experiments further reveal the anomalous nonlinear CEF splitting under magnetic fields. We have grown single crystals of YbOCl, the nonmagnetic LuOCl, and the diluted magnetic Lu_{0.86}Yb_{0.14}OCl to make a completely comparative investigation. Polarized Raman spectra on the samples at 1.8 K allow us to clearly assign all the Raman-active phonon modes and explicitly identify the CEF excitations in YbOCl. The CEF excitations are further examined using temperature-dependent Raman measurements and careful symmetry analysis based on Raman tensors related to CEF excitations. By applying the CEF Hamiltonian to the experimentally determined CEF excitations, we extract the CEF parameters and eventually determine the CEF wave functions. The study experimentally pins down the CEF excitations in the Kitaev compound YbOCl and sets a foundation for understanding its finite-temperature magnetism and exploring the possible nontrivial spin ground state.

In this work, we demonstrate a simple yet effective procedure for identifying the exotic tetrahedral triple-Q magnetic ordering over the domain averaged collinear single-Q magnetic state for the triangular antiferromagnet Co1/3NbS2. Our density functional theory (DFT) calculations show that the hyperfine magnetic field exhibits a significant anisotropic dependence on the angle between the spin direction and the c-axis, which means that the atoms with different spin directions in the triple-Q state can be divided into two subgroups with a ratio of 1:3 that can be distinguished by local probing methods. Mossbauer spectroscopy measurements on (Co0.99 57Fe0.01)1/3NbS2 crystals confirm exactly our DFT predictions and thus give possible evidence of the tetrahedral triple-Q magnetic state. These results provide important insights into the magnetic ground state of Co1/3NbS2, which shall shed light on the understanding of the anomalous Hall effect in Co1/3NbS2 and other related materials.

YbOCl is a representative member of the van der Waals layered honeycomb rare-earth chalcohalide RChX (R = rare earth; Ch = O, S, Se, and Te; and X = F, Cl, Br, and I) family reported recently. Its spin ground state remains to be explored experimentally. We grew high-quality single crystals of YbOCl and conducted comprehensive thermodynamic, elastic, and inelastic neutron scattering experiments down to 50 mK. The experiments reveal an antiferromagnetic phase below 1.3 K which is identified as a spin ground state with an intralayer ferromagnetic and interlayer antiferromagnetic ordering. By applying sophisticated numerical techniques to a honeycomb (nearest-neighbor)–triangle (next-nearest-neighbor) model Hamiltonian which accurately describes the highly anisotropic spin system, we are able to simulate the experiments well and determine the diagonal and off-diagonal spin-exchange interactions. The simulations give an antiferromagnetic Kitaev term comparable to the Heisenberg one. The experiments under magnetic fields allow us to establish a magnetic field–temperature phase diagram around the spin ground state. Most interestingly, a relatively small magnetic field (∼0.3 to 3 T) can significantly suppress the antiferromagnetic order, suggesting an intriguing interplay of the Kitaev interaction and magnetic fields in the spin system. The present study provides fundamental insights into the highly anisotropic spin systems and opens a window to look into Kitaev spin physics in a rare-earth-based system. Published by the American Physical Society 2024

LaRu2P2 2 P 2 shares a similar ThCr2Si2-type 2 Si 2-type structure with iron pnictides and also shows a superconductivity of 4 K, but possesses quite different electronic properties. The studies of electronic properties and superconductivity in LaRu2P2 2 P 2 are of particular interest. In this paper, we have conducted a comprehensive Raman scattering study and careful first-principles calculations to explore the electron-phonon coupling (EPC) in LaRu2P2. 2 P 2 . We have grown very high-quality LaRu2P2 2 P 2 superconducting single crystals with a superconducting transition width A T c 0.15 . 15 K. The phonon modes observed in polarized Raman measurements are well assigned with a symmetry analysis. The linewidths of the Raman modes exhibit an anomalous temperature dependence though their Raman frequencies approximately follow the general anharmonic decay process. This allows us to extract the intrinsic phonon broadening caused by EPC, and gives a relatively large EPC constant which is consistent with our first-principles calculations. The estimated Tc c 4.2 . 2 K based on the constant suggests a pairing mechanism associated with EPC in LaRu2P2. 2 P 2 .